The invention described herein may be manufactured, used, sold, imported, and/or licensed by or for the Government of the United States of America without the payment to us of any royalties thereon.
The invention relates to radiation sensors and, more particularly, to a spectrometer to measure an unknown neutron spectrum in outer space.
It is often necessary to quickly, accurately and inexpensively measure neutron spectra in low earth orbits covering several energy ranges. High-energy cosmic rays produce neutrons in the upper atmosphere are a particular concern because such neutrons pose a threat to airborne semiconductor equipment such as the memory devices in flight control equipment. Neutrons threaten these devices by causing bit-flips leading to failures in the flight control and navigational equipment, and thereby endangering the operation of both high-flying aircraft like the Concorde and lower altitude commercial aircraft.
There has been a long-standing need to characterize neutron spectra so that physicists and equipment designers can better predict aircraft upset rates and design systems to avoid catastrophic aircraft failures. The general operating principle for neutron spectrometers is that neutrons interact with certain atoms to produce recoil protons that travel in relatively straight lines, as described in Kronenberg, S. and H. Murphy, xe2x80x9cEnergy Spectrum of Protons Emitted From a Fast-Neutron-Irradiated Hydrogenous Materialxe2x80x9d, Radiation Research 12, 728-735 1960.
Several types of detectors that have been used in prior art neutron spectrometers of this type to measure the recoil protons. One of the earliest applications described in Kronenberg, S., xe2x80x9cFast Neutron Spectroscope for Measurements in a High Intensity Time Dependent Neutron Environmentxe2x80x9d, International Symposium on Nuclear Electronicsxe2x80x9d, Paris France, Comptes Rendus, May 1964. That device utilized a scintillation counter, consisting of cesium iodide and a photomultiplier and solid state devices. A variation of that approach employing a PMOS transistor was described in Kronenberg, S. and G. J. Brucker, xe2x80x9cThe Use of Hydrogenous Material for Sensitizing PMOS Dosimeters to Neutronsxe2x80x9d, IEEE Trans. Nucl. Sci., Vol. 42, No. 1, February 1995.
One significant limitation of these prior art devices is that they can only count protons and can neither characterize neutron spectra nor generate the original neutron spectra. These prior art neutron spectrometers suffered from a number of other disadvantages, limitations and shortcomings because of their size, weight cost and complex circuitry, making them unsuitable for use in spacecraft and other airborne applications. In fact, the NASA Goddard Space Flight Center recently requested proposals for the measurement of high-energy spectra with a spectrometer on-board a satellite or the Shuttle spacecraft.
To overcome the prior art""s inability to characterize neutron spectra, as well as disadvantages, limitations and shortcomings of size, weight, cost and complex circuitry, the present invention fulfills this long-standing need with a simplified, compact and inexpensive neutron spectrometer detector. The neutron spectrometer detector employs a thin depletion layer, silicon, solid state detector as a proton counter in an instrument that converts a distribution of neutrons to one of recoil protons. The present invention""s neutron spectrometer uses computer technology to allow for greater and quicker data reduction and provides the added capability of characterizing neutron spectra by unfolding proton recoil spectra into the original neutron spectrum that produced the proton particles.
The preferred embodiment is flat neutron spectrometer monitor with an arrangement of detectors, converters and absorbers housed within a chamber. The advantages of low weight, compact size, simplified operation and increased data reduction allow the present invention""s neutron spectrometer to fulfill the long-standing need for measuring high-energy spectra, without suffering from the disadvantages, limitations and shortcomings of prior art devices. A dodecahedron embodiment of the neutron spectrometer with the detectors, converters and absorbers housed within a sphere is also described.
It is one object of the neutron spectrometer to measure neutron spectra on land or in the laboratory.
It is another object of the neutron spectrometer to measure neutron spectra covering several energy ranges from 1 to 250 MeV.
It is an additional object of the neutron spectrometer to convert a distribution of neutrons to one of recoil protons sorted into numerous energy bins where they are counted and the original neutron spectrum is generated by software.
To attain these and other objects and advantages, the neutron spectrometer of the present invention provides a series of substrates covered by a solid-state detector stacked on an absorbing layer. In this arrangement, as many as 12 substrates that convert neutrons to protons, are covered by a layer of absorbing material, acting as a proton absorber, with the detector placed within the layer to count protons passing through the absorbing layer. By using 12 detectors the present invention covers the range of neutron energies. The present invention encompasses a preferred dodecahedron spectrometer, and other shapes are also possible.
The dodecahedron embodiment of the present invention""s neutron spectrometer comprises a solid, polyethylene dodecahedron assembly with its 12 surface facets covered by a solid-state detector stacked on an absorbing layer. In this arrangement, each of 12 surface pentagon-shaped facets provides a polyethylene substrate to convert neutrons to protons, covered by a layer of absorbing material, acting as a proton absorber, with the detector stacked on the absorbing layer to count protons passing through the absorbing layer. The dodecahedron assembly is housed concentrically within a titanium spherical shell that serves as an outer shield. The dodecahedron embodiment is lightweight and therefore would be suitable for airborne and satellite applications.